diff --git a/compress_generate.go b/compress_generate.go
new file mode 100644
index 00000000..1a301e9f
--- /dev/null
+++ b/compress_generate.go
@@ -0,0 +1,184 @@
+//+build ignore
+
+// compression_generate.go is meant to run with go generate. It will use
+// go/{importer,types} to track down all the RR struct types. Then for each type
+// it will look to see if there are (compressible) names, if so it will add that
+// type to compressionLenHelperType and comressionLenSearchType which "fake" the
+// compression so that Len() is fast.
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"go/format"
+	"go/importer"
+	"go/types"
+	"log"
+	"os"
+)
+
+var packageHdr = `
+// *** DO NOT MODIFY ***
+// AUTOGENERATED BY go generate from compress_generate.go
+
+package dns
+
+`
+
+// getTypeStruct will take a type and the package scope, and return the
+// (innermost) struct if the type is considered a RR type (currently defined as
+// those structs beginning with a RR_Header, could be redefined as implementing
+// the RR interface). The bool return value indicates if embedded structs were
+// resolved.
+func getTypeStruct(t types.Type, scope *types.Scope) (*types.Struct, bool) {
+	st, ok := t.Underlying().(*types.Struct)
+	if !ok {
+		return nil, false
+	}
+	if st.Field(0).Type() == scope.Lookup("RR_Header").Type() {
+		return st, false
+	}
+	if st.Field(0).Anonymous() {
+		st, _ := getTypeStruct(st.Field(0).Type(), scope)
+		return st, true
+	}
+	return nil, false
+}
+
+func main() {
+	// Import and type-check the package
+	pkg, err := importer.Default().Import("github.com/miekg/dns")
+	fatalIfErr(err)
+	scope := pkg.Scope()
+
+	domainTypes := map[string]bool{}  // Types that have a domain name in them (either comressible or not).
+	cdomainTypes := map[string]bool{} // Types that have a compressible domain name in them (subset of domainType)
+	for _, name := range scope.Names() {
+		o := scope.Lookup(name)
+		if o == nil || !o.Exported() {
+			continue
+		}
+		st, _ := getTypeStruct(o.Type(), scope)
+		if st == nil {
+			continue
+		}
+		if name == "PrivateRR" {
+			continue
+		}
+
+		if scope.Lookup("Type"+o.Name()) == nil && o.Name() != "RFC3597" {
+			log.Fatalf("Constant Type%s does not exist.", o.Name())
+		}
+
+		for i := 1; i < st.NumFields(); i++ {
+			if _, ok := st.Field(i).Type().(*types.Slice); ok {
+				if st.Tag(i) == `dns:"domain-name"` {
+					domainTypes[o.Name()] = true
+				}
+				if st.Tag(i) == `dns:"cdomain-name"` {
+					cdomainTypes[o.Name()] = true
+					domainTypes[o.Name()] = true
+				}
+				continue
+			}
+
+			switch {
+			case st.Tag(i) == `dns:"domain-name"`:
+				domainTypes[o.Name()] = true
+			case st.Tag(i) == `dns:"cdomain-name"`:
+				cdomainTypes[o.Name()] = true
+				domainTypes[o.Name()] = true
+			}
+		}
+	}
+
+	b := &bytes.Buffer{}
+	b.WriteString(packageHdr)
+
+	// compressionLenHelperType - all types that have domain-name/cdomain-name can be used for compressing names
+
+	fmt.Fprint(b, "func compressionLenHelperType(c map[string]int, r RR) {\n")
+	fmt.Fprint(b, "switch x := r.(type) {\n")
+	for name, _ := range domainTypes {
+		o := scope.Lookup(name)
+		st, _ := getTypeStruct(o.Type(), scope)
+
+		fmt.Fprintf(b, "case *%s:\n", name)
+		for i := 1; i < st.NumFields(); i++ {
+			out := func(s string) { fmt.Fprintf(b, "compressionLenHelper(c, x.%s)\n", st.Field(i).Name()) }
+
+			if _, ok := st.Field(i).Type().(*types.Slice); ok {
+				switch st.Tag(i) {
+				case `dns:"domain-name"`:
+					fallthrough
+				case `dns:"cdomain-name"`:
+					// For HIP we need to slice over the elements in this slice.
+					fmt.Fprintf(b, `for i := range x.%s {
+						compressionLenHelper(c, x.%s[i])
+					}
+`, st.Field(i).Name(), st.Field(i).Name())
+				}
+				continue
+			}
+
+			switch {
+			case st.Tag(i) == `dns:"cdomain-name"`:
+				fallthrough
+			case st.Tag(i) == `dns:"domain-name"`:
+				out(st.Field(i).Name())
+			}
+		}
+	}
+	fmt.Fprintln(b, "}\n}\n\n")
+
+	// compressionLenSearchType - search cdomain-tags types for compressible names.
+
+	fmt.Fprint(b, "func compressionLenSearchType(c map[string]int, r RR) (int, bool) {\n")
+	fmt.Fprint(b, "switch x := r.(type) {\n")
+	for name, _ := range cdomainTypes {
+		o := scope.Lookup(name)
+		st, _ := getTypeStruct(o.Type(), scope)
+
+		fmt.Fprintf(b, "case *%s:\n", name)
+		j := 1
+		for i := 1; i < st.NumFields(); i++ {
+			out := func(s string, j int) {
+				fmt.Fprintf(b, "k%d, ok%d := compressionLenSearch(c, x.%s)\n", j, j, st.Field(i).Name())
+			}
+
+			// There are no slice types with names that can be compressed.
+
+			switch {
+			case st.Tag(i) == `dns:"cdomain-name"`:
+				out(st.Field(i).Name(), j)
+				j++
+			}
+		}
+		k := "k1"
+		ok := "ok1"
+		for i := 2; i < j; i++ {
+			k += fmt.Sprintf(" + k%d", i)
+			ok += fmt.Sprintf(" && ok%d", i)
+		}
+		fmt.Fprintf(b, "return %s, %s\n", k, ok)
+	}
+	fmt.Fprintln(b, "}\nreturn 0, false\n}\n\n")
+
+	// gofmt
+	res, err := format.Source(b.Bytes())
+	if err != nil {
+		b.WriteTo(os.Stderr)
+		log.Fatal(err)
+	}
+
+	f, err := os.Create("zcompress.go")
+	fatalIfErr(err)
+	defer f.Close()
+	f.Write(res)
+}
+
+func fatalIfErr(err error) {
+	if err != nil {
+		log.Fatal(err)
+	}
+}
diff --git a/dns_test.go b/dns_test.go
index ad68533f..dbfe2532 100644
--- a/dns_test.go
+++ b/dns_test.go
@@ -310,6 +310,23 @@ func TestMsgLengthCompressionMalformed(t *testing.T) {
 	m.Len() // Should not crash.
 }
 
+func TestMsgCompressLength2(t *testing.T) {
+	msg := new(Msg)
+	msg.Compress = true
+	msg.SetQuestion(Fqdn("bliep."), TypeANY)
+	msg.Answer = append(msg.Answer, &SRV{Hdr: RR_Header{Name: "blaat.", Rrtype: 0x21, Class: 0x1, Ttl: 0x3c}, Port: 0x4c57, Target: "foo.bar."})
+	msg.Extra = append(msg.Extra, &A{Hdr: RR_Header{Name: "foo.bar.", Rrtype: 0x1, Class: 0x1, Ttl: 0x3c}, A: net.IP{0xac, 0x11, 0x0, 0x3}})
+	predicted := msg.Len()
+	buf, err := msg.Pack()
+	if err != nil {
+		t.Error(err)
+	}
+	if predicted != len(buf) {
+		t.Errorf("predicted compressed length is wrong: predicted %s (len=%d) %d, actual %d",
+			msg.Question[0].Name, len(msg.Answer), predicted, len(buf))
+	}
+}
+
 func TestToRFC3597(t *testing.T) {
 	a, _ := NewRR("miek.nl. IN A 10.0.1.1")
 	x := new(RFC3597)
diff --git a/msg.go b/msg.go
index c74e724f..55ba05fc 100644
--- a/msg.go
+++ b/msg.go
@@ -9,6 +9,7 @@
 package dns
 
 //go:generate go run msg_generate.go
+//go:generate go run compress_generate.go
 
 import (
 	crand "crypto/rand"
@@ -262,7 +263,9 @@ func packDomainName(s string, msg []byte, off int, compression map[string]int, c
 				bsFresh = true
 			}
 			// Don't try to compress '.'
-			if compress && roBs[begin:] != "." {
+			// We should only compress when compress it true, but we should also still pick
+			// up names that can be used for *future* compression(s).
+			if compression != nil && roBs[begin:] != "." {
 				if p, ok := compression[roBs[begin:]]; !ok {
 					// Only offsets smaller than this can be used.
 					if offset < maxCompressionOffset {
@@ -892,10 +895,8 @@ func (dns *Msg) String() string {
 func (dns *Msg) Len() int {
 	// We always return one more than needed.
 	l := 12 // Message header is always 12 bytes
-	var compression map[string]int
-	if dns.Compress {
-		compression = make(map[string]int)
-	}
+	compression := map[string]int{}
+
 	for i := 0; i < len(dns.Question); i++ {
 		l += dns.Question[i].len()
 		if dns.Compress {
@@ -991,97 +992,6 @@ func compressionLenSearch(c map[string]int, s string) (int, bool) {
 	return 0, false
 }
 
-// TODO(miek): should add all types, because the all can be *used* for compression. Autogenerate from msg_generate and put in zmsg.go
-func compressionLenHelperType(c map[string]int, r RR) {
-	switch x := r.(type) {
-	case *NS:
-		compressionLenHelper(c, x.Ns)
-	case *MX:
-		compressionLenHelper(c, x.Mx)
-	case *CNAME:
-		compressionLenHelper(c, x.Target)
-	case *PTR:
-		compressionLenHelper(c, x.Ptr)
-	case *SOA:
-		compressionLenHelper(c, x.Ns)
-		compressionLenHelper(c, x.Mbox)
-	case *MB:
-		compressionLenHelper(c, x.Mb)
-	case *MG:
-		compressionLenHelper(c, x.Mg)
-	case *MR:
-		compressionLenHelper(c, x.Mr)
-	case *MF:
-		compressionLenHelper(c, x.Mf)
-	case *MD:
-		compressionLenHelper(c, x.Md)
-	case *RT:
-		compressionLenHelper(c, x.Host)
-	case *RP:
-		compressionLenHelper(c, x.Mbox)
-		compressionLenHelper(c, x.Txt)
-	case *MINFO:
-		compressionLenHelper(c, x.Rmail)
-		compressionLenHelper(c, x.Email)
-	case *AFSDB:
-		compressionLenHelper(c, x.Hostname)
-	case *SRV:
-		compressionLenHelper(c, x.Target)
-	case *NAPTR:
-		compressionLenHelper(c, x.Replacement)
-	case *RRSIG:
-		compressionLenHelper(c, x.SignerName)
-	case *NSEC:
-		compressionLenHelper(c, x.NextDomain)
-		// HIP?
-	}
-}
-
-// Only search on compressing these types.
-func compressionLenSearchType(c map[string]int, r RR) (int, bool) {
-	switch x := r.(type) {
-	case *NS:
-		return compressionLenSearch(c, x.Ns)
-	case *MX:
-		return compressionLenSearch(c, x.Mx)
-	case *CNAME:
-		return compressionLenSearch(c, x.Target)
-	case *DNAME:
-		return compressionLenSearch(c, x.Target)
-	case *PTR:
-		return compressionLenSearch(c, x.Ptr)
-	case *SOA:
-		k, ok := compressionLenSearch(c, x.Ns)
-		k1, ok1 := compressionLenSearch(c, x.Mbox)
-		if !ok && !ok1 {
-			return 0, false
-		}
-		return k + k1, true
-	case *MB:
-		return compressionLenSearch(c, x.Mb)
-	case *MG:
-		return compressionLenSearch(c, x.Mg)
-	case *MR:
-		return compressionLenSearch(c, x.Mr)
-	case *MF:
-		return compressionLenSearch(c, x.Mf)
-	case *MD:
-		return compressionLenSearch(c, x.Md)
-	case *RT:
-		return compressionLenSearch(c, x.Host)
-	case *MINFO:
-		k, ok := compressionLenSearch(c, x.Rmail)
-		k1, ok1 := compressionLenSearch(c, x.Email)
-		if !ok && !ok1 {
-			return 0, false
-		}
-		return k + k1, true
-	case *AFSDB:
-		return compressionLenSearch(c, x.Hostname)
-	}
-	return 0, false
-}
-
 // Copy returns a new RR which is a deep-copy of r.
 func Copy(r RR) RR { r1 := r.copy(); return r1 }
 
diff --git a/zcompress.go b/zcompress.go
new file mode 100644
index 00000000..1871d4ae
--- /dev/null
+++ b/zcompress.go
@@ -0,0 +1,119 @@
+// *** DO NOT MODIFY ***
+// AUTOGENERATED BY go generate from compress_generate.go
+
+package dns
+
+func compressionLenHelperType(c map[string]int, r RR) {
+	switch x := r.(type) {
+	case *AFSDB:
+		compressionLenHelper(c, x.Hostname)
+	case *RRSIG:
+		compressionLenHelper(c, x.SignerName)
+	case *MD:
+		compressionLenHelper(c, x.Md)
+	case *MX:
+		compressionLenHelper(c, x.Mx)
+	case *TALINK:
+		compressionLenHelper(c, x.PreviousName)
+		compressionLenHelper(c, x.NextName)
+	case *MF:
+		compressionLenHelper(c, x.Mf)
+	case *RP:
+		compressionLenHelper(c, x.Mbox)
+		compressionLenHelper(c, x.Txt)
+	case *RT:
+		compressionLenHelper(c, x.Host)
+	case *SOA:
+		compressionLenHelper(c, x.Ns)
+		compressionLenHelper(c, x.Mbox)
+	case *NSAPPTR:
+		compressionLenHelper(c, x.Ptr)
+	case *CNAME:
+		compressionLenHelper(c, x.Target)
+	case *LP:
+		compressionLenHelper(c, x.Fqdn)
+	case *NSEC:
+		compressionLenHelper(c, x.NextDomain)
+	case *PTR:
+		compressionLenHelper(c, x.Ptr)
+	case *TSIG:
+		compressionLenHelper(c, x.Algorithm)
+	case *MG:
+		compressionLenHelper(c, x.Mg)
+	case *PX:
+		compressionLenHelper(c, x.Map822)
+		compressionLenHelper(c, x.Mapx400)
+	case *SRV:
+		compressionLenHelper(c, x.Target)
+	case *TKEY:
+		compressionLenHelper(c, x.Algorithm)
+	case *MINFO:
+		compressionLenHelper(c, x.Rmail)
+		compressionLenHelper(c, x.Email)
+	case *NAPTR:
+		compressionLenHelper(c, x.Replacement)
+	case *SIG:
+		compressionLenHelper(c, x.SignerName)
+	case *DNAME:
+		compressionLenHelper(c, x.Target)
+	case *HIP:
+		for i := range x.RendezvousServers {
+			compressionLenHelper(c, x.RendezvousServers[i])
+		}
+	case *KX:
+		compressionLenHelper(c, x.Exchanger)
+	case *MB:
+		compressionLenHelper(c, x.Mb)
+	case *MR:
+		compressionLenHelper(c, x.Mr)
+	case *NS:
+		compressionLenHelper(c, x.Ns)
+	}
+}
+
+func compressionLenSearchType(c map[string]int, r RR) (int, bool) {
+	switch x := r.(type) {
+	case *PTR:
+		k1, ok1 := compressionLenSearch(c, x.Ptr)
+		return k1, ok1
+	case *RT:
+		k1, ok1 := compressionLenSearch(c, x.Host)
+		return k1, ok1
+	case *SOA:
+		k1, ok1 := compressionLenSearch(c, x.Ns)
+		k2, ok2 := compressionLenSearch(c, x.Mbox)
+		return k1 + k2, ok1 && ok2
+	case *MB:
+		k1, ok1 := compressionLenSearch(c, x.Mb)
+		return k1, ok1
+	case *MG:
+		k1, ok1 := compressionLenSearch(c, x.Mg)
+		return k1, ok1
+	case *MR:
+		k1, ok1 := compressionLenSearch(c, x.Mr)
+		return k1, ok1
+	case *NS:
+		k1, ok1 := compressionLenSearch(c, x.Ns)
+		return k1, ok1
+	case *MINFO:
+		k1, ok1 := compressionLenSearch(c, x.Rmail)
+		k2, ok2 := compressionLenSearch(c, x.Email)
+		return k1 + k2, ok1 && ok2
+	case *MX:
+		k1, ok1 := compressionLenSearch(c, x.Mx)
+		return k1, ok1
+	case *AFSDB:
+		k1, ok1 := compressionLenSearch(c, x.Hostname)
+		return k1, ok1
+	case *CNAME:
+		k1, ok1 := compressionLenSearch(c, x.Target)
+		return k1, ok1
+	case *MD:
+		k1, ok1 := compressionLenSearch(c, x.Md)
+		return k1, ok1
+	case *MF:
+		k1, ok1 := compressionLenSearch(c, x.Mf)
+		return k1, ok1
+	}
+	return 0, false
+}